1. Field of the Invention
The present invention relates to a lubricant composition, an article, a disk molding stamper, a disk molding apparatus, a disk forming method and a method of forming lubrication coating.
2. Description of the Related Art
Conventionally, a disc substrate for an optical disc is molded by a molding apparatus having a mold 101 as shown in FIG. 15. The mold 101 has for example two mold members made of a steel material such as stainless steel. The mold members are a stationary mold member 102 and a movable mold member 103. The movable mold member 103 can be opened. The stationary mold member 102 has a stationary mirror 121. The stationary mirror 121 has a concave and convex shaped stamper 122 which transfers a concave and convex information signal such as lands/grooves or pits of an optical disc. In the stationary mold member 102, a sprue bush 123 with which resin as a material of a disc substrate is injected is formed.
A disc substrate is molded by the mold 101 for example in the following manner. After the stamper 122 is mounted on the stationary mirror 121, the movable mold member 103 is moved toward the stationary mold member 102. Thereafter, the mold 101 is closed. Resin is injected into a cavity 114 which is a space portion formed between the stationary mold member 102 and the movable mold member 103 through the sprue bush 123. A concave and convex shape formed on the stamper 122 is transferred to the resin. After the concave and convex shape is transferred, the injected resin is cooled. After the injected resin is cooled, the movable mold member 103 is opened. After the movable mold member 103 is opened, resin in the sprue bush 123 and a molded substance as a disc substrate in the sprue bush 123 are ejected by an ejector 130 so that the molded substance is peeled off from the movable mold member 103. After an eject operation of the ejector 130 for the molded substance is finished, the disc substrate is transferred to the outside of the molding apparatus by an unloader (not shown).
In a sequence of molding steps, when molten resin is injected into the cavity 114 through the sprue bush 123, tensile stress radially works due to viscosity of the resin and thermal stress works due to repeated heating and cooling on the stamper 122. On the other hand, tensile stress due to viscosity of resin and thermal stress do not almost work on the stationary mirror 121. Thus, these stresses cause frictional force to work between the stationary mirror 121 and the stamper 122 in the radial direction. As a result, contact portions of the stamper 122 and the stationary mirror 121 wear out.
This wear is called adhesion wear and causes the front surface of the stationary mirror 121 or the rear surface of the stamper 122 to wear out. In other words, the contact surfaces of the stamper 122 and the stationary mirror 121 are microscopically not perfect planes. Thus, the contact surfaces have a concave and convex portion. The concave and convex portion causes the front surface of the disc substrate to be formed in a concave and convex shape and frictional force between the stamper 122 and the stationary mirror 121 to increase. The increased frictional force prevents the stamper 122 to expand and shrink. In the disc substrate formed in such a state, address pits deform, which result in address errors, and roundness of lands/grooves deteriorates, which results in servo errors. If the disc substrate does not satisfy these allowable ranges, the disc substrate is treated as a defective item. As a result, the yield decrease.
Thus, normally, coating is applied to the stationary mirror 121 so as to decrease frictional force of the contact portions of the stamper 122 and stationary mirror 121. The mainstream of the coating is DLC (diamond like carbon) coating of an amorphous carbon film containing hydrogen. The DLC has characteristics similar to those of diamond. The DLC coating can suppress an increase of fictional force due to adhesion wear in comparison with conventional coating of titanium nitride (TiN) or the like. A technology of which a hard film made of DLC is formed on a mold contact surface of a stamper is disclosed in for example Japanese Patent Application Unexamined Publication No. HEI 10-64127, Paragraph [0045].
In recent years, high density optical discs have been developed. For example, a 3.5-inch MO (Magnet Optical) disc accomplishes a record capacity of 2.3 GB using a combination of MSR (Magnetically inducted Super Resolution) technology and land/groove combined record technology. In addition, DVD (Digital Versatile Disc) can record and reproduce data with a high speed record density of 16× speed.
When disc substrates for these high density optical discs are successively molded, since a concave and convex shape transferred by the stamper gradually deteriorates due to the foregoing adhesion wear. Thus, before the characteristics of optical discs exceed the allowable range of the predetermined address error or servo error, it is necessary to abrade the rear surface of the stamper or replace it with good one. Since abrasion or replacement of the stamper takes a time, the productivity remarkably lowers. Thus, it is preferred to reduce as much frictional force which occurs between the stamper and the mold as possible to reduce the abrasion work or replacement work for the stamper.
However, with only the hard film formed between the stamper and the mold, frictional force occurring between the stamper and the mold was not sufficiently decreased. When the foregoing 3.5-inch MO discs were molded for around 20,000 shots, the characteristics of the optical discs exceeded the allowable range of address error.
When oil such as wax or lubrication oil is applied to the stamper contact portion of the mold and/or the mold contact portion of the stamper, frictional force which occurs between the stamper and the mold can be decreased.
However, oil such as wax does not have heat resistance. When oil is used under high temperature environment for a long time, it loses stability. When oil is further heated, moisture is lost therefrom. The moisture adversely affects the stamper and the mold. As a result, the quality of the molded substance deteriorates. In addition, it is difficult to equally apply oil such as wax and lubrication oil on the front surface of the mold. Thus, oil and lubrication oil do not have reproducibility. In addition, since oil such as wax and lubrication oil have high viscosity, they tend to catch foreign matter such as dust. As a result, the quality of the molded substance deteriorates.
Lubricant applied between the stamper and the mold has necessity for the following characteristics to reduce frictional force occurring therebetween. (1) Lubricant does not change in appearance and is dry. (2) Lubricant has solvent resistance. (3) Lubricant is strongly absorbed into the rear surface of the stamper or by a material that composes the signal mirror portion. (4) Lubricant does not change the roughness of a surface on which the treatment agent was coated. (5) Lubricant has heat resistance. (6) Under high temperature and high pressure environment, lubricant maintains frictional reduction effect. Next, the characteristics (1) to (6) will be described in detail.
(1) First, lubricant does not change in appearance and is dry. It is not preferred that the entire rear surface of the stamper be oily and wet because dust in the air adheres to the stamper and foreign matter adheres to the stamper that is handled. In addition, when the stamper is wet, lubricant may be present as liquid on the rear surface. A signal surface may be contaminated with treatment agent of lubricant due to high pressure applied when a disc substrate is molded. As a result, lubricant adversely affects the quality of the disc substrate. In addition, lubricant may adhere to the molding apparatus itself. Thus, it is necessary to keep the front surface treated with lubricant dry.(2) In addition, lubricant has solvent resistance. When the stamper is mounted on the mold of the molding apparatus, the rear surface of the stamper and the signal mirror portion of the mold of the molding apparatus are wiped and cleaned with volatile organic solvent. This wiping and cleaning are ordinarily performed to prevent foreign matter from being transferred to the stamper and the signal record portion of the optical disc substrate and to prevent them from being oil contaminated. Volatile solvent used in this case is for example one of kinds of ketones such as acetone, methyl ethyl ketone (MEK), or toluene or one of kinds of alcohols such as ethyl alcohol or isopropyl alcohol (IPA). Among them, acetone or methyl ethyl ketone (MEK) is mainly used. When they are wiped with such a highly dissolvable solvent, most surface active agents, lubricants, resins, and so forth are dissolved. As a result, they are lost from the coated or treated surface. Thus, it is necessary to select a treatment agent that is not dissolvable in these solvents.(3) In addition, lubricant is strongly absorbed into the rear surface of the stamper or by a material that composes the signal mirror portion. It is necessary to equally and chemically absorb the lubricant on the front surface although it depends on solvability. When one of silicone resins and fluoride resins which have frictional reduction effect is used, they are physically absorbed into the rear surface of the stamper and the signal mirror portion. Thus, when they are wiped, these resins are easily removed. Thus, it is difficult to equally form a film of lubricant. In addition, the treatment agent is transferred to the opposite member. As a result, the effect of the lubricant is not maintained for a long time.(4) In addition, lubricant does not change the roughness of a surface on which the treatment agent was coated. When the surface roughness deteriorates, the deteriorated roughness may be transferred to the stamper and the signal record portion of the optical disc substrate under high temperature and high pressure environment. As a result, they may be adversely affected. Although PTFE (polytetrafluoro-ethylene), graphite/molybdenum disulfide solid lubricants, and so forth have remarkable frictional reduction effect, they often deteriorate the surface roughness. Thus, it is difficult to practically use them. In addition, it is difficult to control solid lubricant for an equal film thickness. Thus, solid lubricant may not be used to mold optical discs that have necessity for large surface roughness. In addition, a graphite type lubricant and a molybdenum disulfide type lubricant are normally powder. Since powder drops from the treatment surface, it may be impossible to use them from a view point of quality.(5) In addition, lubricant has heat resistance. When an optical disc substrate is molded, resin heated at around 400° C. is injected into the stamper. At this point, although the rear surface of the stamper and the signal mirror portion are quickly cooled, since optical disc substrates are successively molded, they are continuously heated at around 200° C. under high pressure. At this environmental temperature, the frictional reduction effect of most of surface-active agents and resins is lost by pyrolysis or the like.(6) In addition, under high temperature and high pressure environment, lubricant maintains frictional reduction effect. It is most important for lubricant which reduces frictional force occurring between the stamper and the mold to satisfy this characteristic. Besides the fabrication of optical discs, the facility operating ratio of the fabrication site is very important from a view point of cost. When the facility is stopped and a maintenance work is performed, human mistake factors such as catching of foreign matter increase. As a result, the yield of optical discs is adversely affected. It is preferred that the facility be continuously operated as long as possible.
However, those in the field of the present invention know that it is very difficult to find lubricant that satisfies the characteristics (1) to (6). Thus, it has been desired to accomplish lubricant that satisfies the characteristics (1) to (6).
In addition, in recent years, it has been strongly desired to improve the productivity of high density optical record mediums such as DVD and quantitatively produce next generation high density optical record mediums such as BD. Under such a circumstance, it has been desired to accomplish lubricant that satisfies the characteristics (1) to (6).